The $24.99-per-year iTunes Match debuted alongside the release of iTunes 10.5.1, which fixed one critical security vulnerability that could let attackers push fake software updates to Windows and Mac users.

First introduced in June by then-CEO Steve Jobs, iTunes Match was later slated by Apple to launch in October, a date it missed.

iTunes Match lets tracks users have "ripped" from a purchased CD or acquired through other means to listen to their music from multiple devices, not only the Windows PC, Mac, iPhone, iPad or iPod Touch where the digital files are stored. Such tracks are automatically uploaded to iCloud, Apple's free syncing service.

iTunes Match doesn't require users to actually upload their music library, but instead the service scans a customer's collection, matches it against the 18 million tracks in Apple's music store, then makes the matches available for instant downloading to a maximum of 10 devices or computers from iCloud.

The matched tracks on Apple's servers are 256Kbps copies, even if the ripped tunes were originally translated to a lower-bitrate version.

Up to 25,000 tracks not purchased via iTunes can be stored on iCloud using iTunes Match; iTunes-bought tunes do not count toward the maximum.

Almost immediately after launching iTunes Match, Apple's website claimed that "Due to overwhelming demand, iTunes Match is not accepting new subscriptions at this time."

However, Computerworld ignored the message, and was able to subscribe to the service and process a medium-sized music collection housed on an iMac.

Contrary to Jobs' contention last June that the matching procedure would take "minutes, not weeks" -- a clear jab at the music lockers run by rivals Amazon and Google -- processing a Computerworld reporter's music library took several hours.

The speed with which iTunes Match matches tracks, then uploads music files to iCloud depends on the size of the user's collection and the speed of his or her Internet connection.

iTunes 10.5.1, which is necessary to subscribe to iTunes Match from a Windows PC or Mac, can be downloaded from Apple's website. As of 2 p.m. ET, 10.5.1 had not made it to Apple's automatic Software Update center.

Besides enabling iTunes Match, the update also patched a vulnerability that could be used by hackers to stage "man-in-the-middle" attacks that appear to be available software updates. According to Apple, Windows users who have not installed the Apple Software Update utility on their PCs are most at risk.

The bug was first disclosed by Argentinian researcher Francisco Amato in 2008, when he produced an attack tool dubbed "Evilgrade" that could inject fake updates for a variety of applications and operating systems, including iTunes, Mac OS X's Software Update and Microsoft's Windows Update.

For $25 annually, iTunes Match stores up to 25,000 tracks not bought from Apple on the free iCloud service.